Conventionally, in automobile outer-panel components and others, tailored blank members have been used that are formed by jointing steel plates different from each other in plate thickness and mechanical characteristics, in order to reduce the weight of a vehicle body and improve collision safety performance. For example, when blanked steel plates (hereinafter referred to as blank members) are butted together and butting portions of these blank members (hereinafter simply referred to as butting portions) are radiated with laser light for butt welding of the blank members to obtain a tailored blank member, if a gap of the butting portions is larger than an allowance value (the magnitude of an allowable gap), a welding defect may occur, such as a pinhole. Thus, management of the gap of the butting portions is extremely important in butt welding of blank members. For this reason, as means of making the gap of the butting portions equal to or smaller than the allowable value, linear precision of each butting portion of each blank member may be increased. However, when each butting portion of each blank member is formed through blanking, precision of a blanking mold (blanking blade) is required to be held high, and operators are therefore required to have high technical skills and strict management of blanking mold is also required.
Although linear precision of each butting portion of each blank member can be increased also through a cutting process using a laser, facility and manufacturing costs are increased. To get around this, Patent document 1 describes a butt jointing device that aligns each butting portion of each blank member and increasing linear precision of the butting portion by applying a butting load to the butting portion with the blank members being butted together and then releasing the butting load. However, in the butt jointing device according to Patent document 1, when relatively large projections and recesses and burrs are present on a butting portion of any blank member, a gap occurs in the butting portion when the butting load is released, thereby possibly causing a welding defect, such as a pinhole, on the butting portion. Moreover, in the butt jointing device of this type, a clamp device that holds the blank members is dedicatedly manufactured in accordance with the shape of each blank member, and therefore there is a problem in general versatility. To get around this, a plurality of clampers each driven by an air cylinder may be disposed along the butting portion, but many air cylinders are required, thereby increasing manufacturing cost.
On the other hand, in some conventional laser welding devices, in order to ensure welding quality, a laser output of a power laser (laser light for actual use in welding) may be regularly measured using a power meter to correct the laser output of an oscillator. However, since measurement takes some time (10 to 60 seconds), no laser output can be measured for each product in view of problems of productivity, and a large number of products with welding defects may have been manufactured at the time of discovering a problem with laser output. Furthermore, some conventional laser welding devices include a pinhole detecting device that determines the presence or absence of a pinhole in a welding bead based on image data shot by a visual sensor, such as a CCD camera. However, since image processing takes some time, pinhole detection and welding cannot be simultaneously performed, and pinhole detection is performed exclusively in an examining process after welding under present circumstances. For this reason, an enormous number of processes are required in the examining process, and a system has been therefore demanded capable of detecting a pinhole simultaneously with welding.
Patent document 1: Japanese Patent Application Publication No. JP-A-2003-275883